Absorption (Laura)

Drug absorption refers to the route or method by which a drug reaches the blood supply. First, we must look at how the drug is administered. There are seven ways this is done; oral administration, absorption through mucous membranes, rectal administration, topical administration, inhalation, injection and implants. Below is an image depicting how a transdermal or topical drug reaches the blood stream and the different methods of injection.

Oral administration refers to drugs taken by the mouth, usually in the form a tablet, capsule, caplet or liquid. A certain amount of orally administered drugs are absorbed in the stomach, but most is absorbed by the upper intestine. Absorption through mucous membranes happens in the nose and mouth. Nasal sprays and sublingual tablets dissolved under the tongue are examples of these. Eye drops also fall into this category. Rectal administered drugs are in the form of suppositories and are taken if the patient cannot swallow or they are unconscious. Topical administration drugs are applied to the skin.
Tadme.jpghey can come in the form of creams or patches. The drug is absorbed through the skin and reaches the blood stream and is generally slow release. Inhalation is another way drugs can be administered. It is absorbed through the cell lining of the respiratory tract and then into the blood stream. Anti-asthmatics are widely used inhaled drugs. Injections are administered directly into a vein, into a muscle, under the surface of the skin, into the spinal cord or into the abdomen.

If the drug is administered orally, it travels through the gastrointestinal tract which consists of the mouth, throat, stomach and intestines. A percentage of the drug, depending on which drug it is and how it is made, can be absorbed through the mucosal membrane of the mouth. The drug moves from the mouth down to the stomach where it can be broken down and digested in the gastric juices. Drugs that are still intact after encountering these acidic juices then make their way to the intestines through the cells lining the gut wall. The drug then passes between the cells lining the blood vessels and enter the blood stream. Through the blood stream the drug meets its next destination, the liver, where it is then metabolized. Shown beside is a rough overview of the pathway a drug takes once entered into the body.

In order for a drug to make it to the liver for metabolism, there are many important characteristics it must possess. If it is not chemically stable enough, it will be broken down by the acids in the stomach. It must also be metabolically stable to pass metabolism in the gastrointestinal tract to avoid the digestive enzymes encountered along the tract as well as in the liver. It is also important to keep in mind the solubility of the drug. The drug will not be able to pass the gut and enter the intestines if it is too polar, or hydrophilic. If it happens to be too hydrophobic, it will end up dissolving in fat globules before it reaches the intestines. It is important to note that molecular weight does not affect the ability of a drug to be absorbed, but rather the solubility as previously described.

Another important characteristic for a drug to contain is amine functional groups. The main importance of these groups is in respect to the drugs ability to bind to its target and to permit good water solubility. Amines are weak bases, and with a pKa between 6 and 8, the ionized and non-ionized form are in equilibrium at blood pH. This allows the drug to cross through cell membranes in their non-ionized form and be water soluble in their ionized form as well as good binding interactions with the target site.